The present invention relates to a method of making directionally solidified (DS) castings and more specifically to a method which reduces the time required to cast a directionally solidified article without reducing the quality of the cast product.
In casting directionally solidified articles from nickel-base superalloys, a mold is commonly positioned on a chill plate which is slowly withdrawn from a furnace to provide for controlled solidification of molten metal in the mold in a manner similar to that disclosed in U.S. Pat. Nos. 3,700,023 and 3,714,977. During various DS casting operations, it has been observed that the mold can be withdrawn from the furnace at speeds of up to about 20 in./hr. to generate acceptable columnar grain structures. The specific speeds at which a particular article is withdrawn from the furnace are governed by the geometry of the article. If it is attempted to use higher speeds, such as 30 in./hr., it has previously been determined that a substantial and very objectionable coarsening of the columnar grains occurs. Attempts to change the temperatures and temperature distribution in the furnace hot zone have proven to be ineffective in permitting the use of faster withdrawal speeds for the production of gas turbine articles such as blades and vanes.
In an effort to increase withdrawal speeds to a rate of 25 in./hr. or faster, U.S. Pat. No. 3,532,155 discloses an apparatus in which the mold and cooling plate are moved through a heat sink which is disposed immediately beneath the furnace. As a still further effort to reduce the time required to form a casting, U.S. Pat. No. 4,190,094 suggests varying the rate of withdrawal of the mold from a furnace as a function of the geometry of the article to be cast and other factors.